Passive reactivity control in a nuclear fission reactor

ABSTRACT

A nuclear reactor includes a passive reactivity control nuclear fuel device located in a nuclear reactor core. The passive reactivity control nuclear fuel device includes a multiple-walled fuel chamber having an outer wall chamber and an inner wall chamber contained within the outer wall chamber. The inner wall chamber is positioned within the outer wall chamber to hold nuclear fuel in a molten fuel state within a high neutron importance region. The inner wall chamber allows at least a portion of the nuclear fuel to move in a molten fuel state to a lower neutron importance region while the molten nuclear fuel remains within the inner wall chamber as the temperature of the nuclear fuel satisfies a negative reactivity feedback expansion temperature condition. A duct contains the multiple-walled fuel chamber and flows a heat conducting fluid through the duct and in thermal communication with the outer wall chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of priority to U.S. ProvisionalPatent Application No. 62/438,323, entitled “Passive Reactivity Controlin a Nuclear Fission Reactor” and filed on Dec. 22, 2016, which isspecifically incorporated by reference herein for all that it disclosesor teaches.

BACKGROUND

A fast spectrum nuclear fission reactor (“a fast neutron reactor”), suchas a sodium fast reactor, generally includes a reactor vessel containinga nuclear reactor core. The nuclear reactor core includes an array ofdevice locations for placement of fuel assembly devices and otherreactor support and control devices. Fissile nuclear fuel within thenuclear reactor core is subjected to neutron collisions that result infission reactions. In a breed-and-burn fast neutron reactor, a fissionchain reaction yields “fast spectrum neutrons” that, in turn, collidewith fertile nuclear fuel, thereby transmuting (“breeding”) the fertilenuclear fuel into fissile nuclear fuel. Liquid coolant flows through thenuclear reactor core, absorbing thermal energy from the nuclear fissionreactions that occur in the nuclear reactor core. The heated coolantthen passes to a heat exchanger and a steam generator, transferring theabsorbed thermal energy to steam in order to drive a turbine thatgenerates electricity. Design of such nuclear reactors involvescombinations of materials, structures, and control systems to achievedesirable operational parameters, including nuclear reactor corestability, efficient thermal generation, long-term structural integrity,etc.

SUMMARY

The described technology provides a fast-acting passive reactivitycontrol nuclear fuel device that functions by thermal expansion of aliquid/molten nuclear fuel under high neutron flux and introduces anegative power feedback for a nuclear fission fast reactor.

A nuclear reactor includes a passive reactivity control nuclear fueldevice located in a nuclear reactor core. The passive reactivity controlnuclear fuel device includes a multiple-walled fuel chamber including anouter wall chamber and an inner wall chamber contained within the outerwall chamber. The inner wall chamber is positioned within the outer wallchamber to hold nuclear fuel in a molten fuel state within a highneutron importance region of the nuclear reactor core. The inner wallchamber is further configured to allow at least a portion of the nuclearfuel to move in a molten fuel state to a lower neutron importance regionof the nuclear reactor core while the molten nuclear fuel remains withinthe inner wall chamber as the internal temperature of the inner wallchamber satisfies a negative reactivity feedback expansion temperaturecondition. A duct contains the multiple-walled fuel chamber and flows aheat conducting fluid through the duct and in thermal communication withthe outer wall chamber. The heat conducting fluid operates as a coolant,and the flow temperature of the heat conducting fluid is typically lessthan the temperatures inside the inner wall chamber during a nuclearreaction.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Otherimplementations are also described and recited herein.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 illustrates a partial-cutaway perspective view of an examplenuclear fission reactor with a nuclear reactor core containing a passivereactivity control nuclear fuel device.

FIG. 2 illustrates a cross-sectional view of an example nuclear reactorcore having an array of locations of nuclear reactor core devices,including passive reactivity control assembly devices.

FIG. 3 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device containing nuclear fuel in asolid fuel state within a high neutron importance region of a nuclearreactor core.

FIG. 4A illustrates a perspective view of a passive reactivity controlnuclear fuel device along a long axis, and FIG. 4B illustrates across-sectional view of the passive reactivity control nuclear fueldevice along the long axis.

FIG. 5 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device containing molten fuel within ahigh neutron importance region of a nuclear reactor core.

FIG. 6 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device containing molten fuel expandingoutside a high neutron importance region of a nuclear reactor core.

FIG. 7 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device containing molten fuel expandingoutside a high neutron importance region of a nuclear reactor core andexpanding an inner wall chamber within which it is contained.

FIG. 8 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device containing nuclear fuel within ahigh neutron importance region of a nuclear reactor core after moltenfuel has densified.

FIG. 9 illustrates a cross-sectional view of an alternative examplepassive reactivity control nuclear fuel device.

FIG. 10 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device in which liquid metal fuel hasexpanded outside a high neutron importance region of a nuclear reactor.

FIG. 11 illustrates a cross-section view of another alternative examplepassive reactivity control nuclear fuel device.

FIG. 12 illustrates a cross-sectional view of another alternativeexample passive reactivity control nuclear fuel device in which liquidmetal fuel has expanded outside a high neutron importance region of anuclear reactor.

FIG. 13 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device including an alternative plenumshape in an inner wall chamber.

DETAILED DESCRIPTIONS

Fast nuclear reactors are typically designed to increase the utilizationefficiency of nuclear fuel (e.g., uranium, plutonium, thorium) and tolimit moderations of neutrons in fission reactions. In manyimplementations, fast nuclear reactors can capture significantly more ofthe energy potentially available in natural uranium, for example, thantypical light-water reactors. Nevertheless, the described technology canbe employed in different types of nuclear reactors, including lightwater reactors.

A particular classification of fast nuclear reactor, referred to as a“breed-and-burn” fast reactor, includes a nuclear reactor capable ofgenerating (“breeding”) more fissile nuclear fuel than it consumes. Forexample, the neutron economy is high enough to breed more fissilenuclear fuel from fertile nuclear reactor fuel, such as uranium-238nuclear or thorium-232 fuel, than it burns. The “burning” is referred toas “burnup” or “fuel utilization” and represents a measure of how muchenergy is extracted from the nuclear fuel. Higher burnup typicallyreduces the amount of nuclear waste remaining after the nuclear fissionreaction terminates.

Another particular classification of a fast nuclear reactor is based onthe type of nuclear fuel used in the nuclear fission reaction. A metalfuel fast nuclear reactor employs a metal nuclear fuel, which has anadvantage of high heat conductivity and a faster neutron spectrum thanin ceramic-fueled fast reactors. Metal fuels can exhibit a high fissileatom density and are normally alloyed, although pure uranium metal hasbeen used in some implementations. In a fast nuclear reactor, minoractinides produced by neutron capture of uranium and plutonium can beused as a metal fuel. A metal actinide fuel is typically an alloy ofzirconium, uranium, plutonium, and minor actinides.

FIG. 1 illustrates a partial-cutaway perspective view of an examplenuclear fission reactor 100 with a nuclear reactor core 102 containingone or more passive reactivity control nuclear fuel devices, such as apassive reactivity control nuclear fuel device 104. Other elementswithin the nuclear reactor core 102 include nuclear fuel assemblydevices (such as a nuclear fuel assembly device 106) and movablereactivity control assembly devices (such as a movable reactivitycontrol assembly device 108). Certain structures of the example nuclearfission reactor 100 have been omitted, such as coolant circulationloops, coolant pumps, heat exchangers, reactor coolant system, etc., inorder to simplify the drawing. Accordingly, it should be understood thatthe example nuclear fission reactor 100 may include different and/oradditional structures not shown in FIG. 1.

Implementations of the example nuclear fission reactor 100 may be sizedfor any application, as desired. For example, various implementations ofthe example nuclear fission reactor 100 may be used in low power (˜5Mega Watt thermal) to around 1000 Mega Watt thermal) applications andlarge power (around 1000 Mega Watt thermal and above) applications, asdesired. In one implementation, the example nuclear fission reactor 100is a fast spectrum nuclear fission reactor having an average neutronenergy of greater than or equal to 0.1 MeV, although otherconfigurations are contemplated. It should be understood, however, thatthe described technology can be employed in different types of nuclearreactors, including light water reactors.

Some of the structural components of the nuclear reactor core 102 may bemade of refractory metals, such as tantalum (Ta), tungsten (W), rhenium(Re), or carbon composites, ceramics, or the like. These materials maybe selected to address the high temperatures at which the nuclearreactor core 102 typically operates. Structural characteristics of thesematerials, including creep resistances, mechanical workability,corrosion resistance, etc., may also be relevant to selection. Suchstructural components define an array of device locations within thenuclear reactor core 102.

The nuclear reactor core 102 is disposed in a reactor vessel 110containing a pool of heat conducting fluid, such as a coolant. Forexample, in various implementations, a reactor coolant system (notshown) includes a pool of liquid sodium coolant (not shown) disposed inthe reactor vessel 110. In such cases, the nuclear reactor core 102 issubmerged in the pool of liquid sodium coolant within the reactor vessel110. The reactor vessel 110 is surrounded by a containment vessel 116that helps prevent loss of the liquid sodium coolant in the unlikelycase of a leak from the reactor vessel 110. In alternativeimplementations, liquid coolant can flow through coolant loopsthroughout the nuclear fission reactor 100.

The nuclear reactor core 102 contains the array of device locations forreceiving various reactor core devices, such as nuclear fuel assemblydevices, movable reactivity control assembly devices, and passivereactivity control fuel assembly devices within the central core region112. An in-vessel handling system 114 is positioned near the top of thereactor vessel 110 and is configured, under control of a reactivitycontrol system (not shown), to shuffle individual reactor core devicesin and/or out of the device locations within the nuclear reactor core102. Some reactor core devices may be removable from the nuclear reactorcore 102, while other reactor core devices may not be removable from thenuclear reactor core 102.

The nuclear reactor core 102 can include a neutron source and a largernuclear fission reaction region. The neutron source provides thermalneutrons to initiate a fission reaction in the fissile nuclear fuel. Thelarger nuclear fission reaction region may contain thorium (Th) oruranium (U) fuel and functions on the general principles of fast neutronspectrum fission breeding.

In one implementation, the nuclear fuel within a nuclear fuel assemblydevice may be contained within fissile nuclear fuel assembly devices orfertile nuclear fuel assembly devices. The difference between fissilenuclear fuel assembly devices or fertile nuclear fuel assembly devicesis effectively the enrichment level of the nuclear fuel, which canchange over time within the nuclear reactor core 102. Structurally,fissile nuclear fuel assembly devices or fertile nuclear fuel assemblydevices can be identical in some implementations.

The nuclear fuel assembly device 106 in the nuclear reactor core 102 caninclude a solid hexagonal duct or tube surrounding a plurality of fuelelements, such as fuel pins, which are organized into the nuclear fuelassembly device 106. Non-hexagonal ducts may also be used in someimplementations. The ducts in a nuclear fuel assembly device 106 allowcoolant to flow past the fuel pins through interstitial gaps betweenadjacent duct walls. Each duct also allows individual assemblyorificing, provides structural support for the fuel bundle, andtransmits handling loads from a handling socket to an inlet nozzle. Fuelpins typically consist of multiple nuclear fuel rods (such as uranium,plutonium or thorium) surrounded by a liner and cladding (and sometimesan additional barrier), which prevents radioactive material fromentering the coolant stream. Individual pins of a nuclear fuel assemblydevice 106 in the nuclear reactor core 102 can contain fissile nuclearfuel or fertile nuclear fuel depending on the original nuclear fuel rodmaterial inserted into the pin and the state of breeding within the pin.

The movable reactivity control assembly device 108 can be inserted intoand/or removed from the central core region 112 by the in-vesselhandling system 114 to provide real-time control of the fission process,balancing the needs of keeping the fission chain reaction active whilepreventing the fission chain reaction from accelerating beyond control.The state of a fission chain reaction is represented by an effectivemultiplication factor, k, which indicates the total number of fissionevents during successive cycles of the chain reaction. When a reactor isin a steady state (i.e., each individual fission event triggers exactlyone subsequent fission event), k equals 1. If k>1, the reactor issupercritical and the reaction rate will accelerate. If k<1, the reactoris subcritical and the fission rate will decrease. Conditions within thecentral core region 112 change over time. Hence, movable reactivitycontrol assemblies may be used to adjust the multiplication factor ofthe fission chain reaction as conditions change. Such assemblies can bemoved to and from different locations in the nuclear reactor core toinfluence the multiplication factor of the fission chain reaction. Inaddition, the axial position (e.g., up/down) position of such assembliescan also be adjusted to influence the multiplication factor of thefission chain reaction.

The movable reactivity control assembly device 108 is a highly effectiveneutron absorbing mechanical structure that can be actively insertedinto or removed from the central core region 112 while the fissionprocess is occurring. A movable reactivity control assembly deviceincludes chemical elements of a sufficiently high neutron capturecross-section to absorb neutrons in the energy range of the nuclearfission reaction, as measured by its absorption cross-section. As such,the movable reactivity control assembly device 108 influences the numberof neutrons available to cause a fission reaction within the nuclearreactor core 102, thereby controlling the fission rate of the fissilenuclear fuel within the nuclear reactor core 102. Example materials usedin movable reactivity control assembly devices of the nuclear fissionreactor 100 include without limitation boron carbide or an alloy ofsilver, indium, and cadmium, europium, or a hafnium-hydride. Bycontrolling the portion of the movable reactivity control assemblydevice 108 (as well as the number of movable reactivity controlassemblies) that interacts with the fission reaction within the centralcore region 112, the multiplication factor can be tuned to maintainreactor criticality. Accordingly, a movable reactivity control assemblydevice 108 represents an adjustable parameter for controlling thenuclear fission reaction.

The passive reactivity control nuclear fuel device 104 contains nuclearfuel material in a solid fuel state that can achieve a molten fuel statewhen the nuclear fuel material temperature exceeds the meltingtemperature of the nuclear fuel material (e.g., satisfying a nuclearfuel melting temperature condition). Example nuclear fuel material mayinclude fuel salts, eutectics, pure metals, etc. In someimplementations, the nuclear fuel material will be alloyed with abonding/carrier material (e.g., Mg) such that the alloyed nuclear fueland bonding/carrier material melts at the same temperature and time,although other implementations may involve the nuclear fuel materialmelting before the bonding/carrier material as temperature rises.

The molten fuel within the passive reactivity control nuclear fueldevice 104 can then move within the passive reactivity control nuclearfuel device 104. By moving in and out of a high neutron importanceregion of the nuclear reactor core 102, the molten nuclear fuel materialcan increase or decrease, respectively, the reactivity within thenuclear reactor core 102. Neutron importance represents a magnitude ofthe contribution of a neutron to power generated by a nuclear reactor.

When the molten fuel satisfies a negative reactivity feedback fuelexpansion temperature condition (e.g., a temperature high enough tocause the expansion of the nuclear fuel to move a portion of the moltenfuel into a lower neutron importance region of the nuclear reactor core102), the reduction in volume of fissile material in the high neutronimportance region provides negative reactivity feedback in the nuclearreactor core 102. For example, when fissile nuclear fuel within thepassive reactivity control nuclear fuel device 104 moves into a lowerneutron importance region of the nuclear reactor core 102 (such asthrough thermal expansion of molten fuel as temperature increases),reactivity within the nuclear reactor core 102 decreases.

Movement of the molten fuel within the passive reactivity controlnuclear fuel device influences the reactivity of the nuclear reactorcore 102. Passive reactivity control nuclear fuel devices can also bemoved to and from different locations in the nuclear reactor core 102 toinfluence the reactivity of the nuclear reactor core 102. In addition,the axial position (e.g., up/down) position of such assemblies can alsobe adjusted to influence the reactivity of the nuclear reactor core 102.

It should be understood that the molten fuel can also act as a neutronpoison within the nuclear reactor. Accordingly, moving molten fuel outof a high neutron importance region and into a lower neutron importanceregion of a nuclear reactor can also provide some level of positivereactivity feedback. Nevertheless, the passive reactivity controlnuclear fuel device 104 can be designed such that the negativereactivity feedback of moving the molten fuel to a lower neutronimportance region exceeds the positive reactivity feedback of removingthe molten fuel (as a poison) from the high neutron importance region.

In contrast, when the molten fuel no longer satisfies a negativereactivity feedback fuel expansion temperature condition (e.g., atemperature that is no longer high enough to cause the expansion of thenuclear fuel to move a portion of the molten fuel into a lower neutronimportance region of the nuclear reactor core 102), the increase involume of fissile material in the high neutron importance regionprovides positive reactivity feedback in the nuclear reactor core 102.For example, when fissile nuclear fuel within the passive reactivitycontrol nuclear fuel device 104 moves back into the high neutronimportance region of the nuclear reactor core 102 (such as throughdensification of molten fuel as temperature decreases), reactivitywithin the nuclear reactor core 102 increases. Accordingly, in oneimplementation, a passive reactivity control nuclear fuel device, suchas the passive reactivity control nuclear fuel device 104, can providenegative feedback to the fission reaction as the temperature of thenuclear fuel material increases and positive feedback to the fissionreaction as the temperature of the nuclear fuel material decreases.

FIG. 2 illustrates a cross-sectional view of an example nuclear reactorcore 202 having an array of locations (such as device location 204) ofnuclear reactor core devices, including passive reactivity controlassembly devices. It should be understood that a fast nuclear reactorcore typically has more device locations and devices than shown in theexample core of FIG. 2, but a reduced number of device locations anddevices is shown to facilitate description and illustration. Each deviceis inserted into a structurally-defined device location within thearray. Reflector devices, such as a replaceable radiation reflectordevice at the device location 204, and permanent radiation reflectormaterial 214 are positioned at the boundary of the central reactor coreregion to reflect neutrons back into the central reactor core region.

Nuclear fuel assembly devices, such as a nuclear fuel assembly device206 and a nuclear fuel assembly device 208, occupy the majority of thedevice locations in the nuclear reactor core 202. As the nuclearreaction progresses, an atom of fertile nuclear fuel can be converted ortransmuted to fissile nuclear fuel by the capture of a neutron within acertain energy range. For example, a fertile nucleus, such as a U-238nucleus, can capture fast neutrons and be transmuted to a fissilenucleus, such as Pu-239, by beta-decay. Meanwhile, in this case, thePu-239 nucleus can capture a neutron, resulting in a fission reactionthat yields multiple fast neutrons. This neutron multiplication witheach fission reaction provides enough neutrons for the transmutation ofnew fissile nuclear fuel from the fertile nuclear fuel. As such, thefission reaction drives the breeding of new fissile nuclear fuel fromfertile nuclear fuel that it consumes. The reactivity of the fissionreaction can be controlled to some extent by one or more movablereactivity control assembly devices, such as a movable reactivitycontrol assembly device 210.

By introducing passive reactivity control nuclear fuel devices, such aspassive reactivity control nuclear fuel device 212 into the nuclearreactor core 202, the reactivity of the fission reaction can bedecreased after the temperature of the nuclear fuel satisfies a negativereactivity feedback expansion temperature condition (e.g., thetemperature of the fuel has exceeded a particular temperature threshold)by thermally expanding molten fuel to move into a lower neutronimportance region of the nuclear reactor core 202. In contrast, thereactivity of the fission reaction can be increased after thetemperature of the nuclear fuel fails to satisfy a negative reactivityfeedback expansion temperature condition (e.g., the temperature of thefuel has decreased below a particular temperature threshold) bydensification of molten fuel to move back into the high neutronimportance region of the nuclear reactor core 202. In oneimplementation, eighteen passive reactivity control nuclear fuel devicesmay be employed throughout the nuclear reactor core 202, althoughdifferent reactor designs may benefit from a greater or lesser number ofpassive reactivity control nuclear fuel devices.

FIG. 3 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device 300 containing nuclear fuel 318in a solid fuel state within a high neutron importance region of anuclear reactor core. The passive reactivity control nuclear fuel device300 includes a duct 304 through which a heat conducting fluid (e.g.,coolant 306, such as molten sodium) can flow. The duct 304 may bemanufactured from HT9 stainless steel although other materials may beemployed.

The duct 304 also contains a multiple-walled fuel chamber 308, which isan example of a multiple-walled fuel chamber. In one implementation, anouter wall chamber 310 of the multiple-walled fuel chamber 308 primarilyincludes HT9 stainless steel, although other materials may be employed,at least in part. Further, in one implementation, an inner wall chamber312 of the multiple-walled fuel chamber 308 primarily includesmolybdenum, although other materials may be employed, at least in part.In one implementation, the inner wall chamber 312 is fitted with one ormore thermally conductive contacts 314, which may also be formed fromHT9 stainless steel or other materials. The contacts 314 can improvethermal communication between the inner wall chamber 312 and the outerwall chamber 310 (and therefore, the coolant 306) as one or more of thecontacts 314 approach and/or physically contact the outer wall chamber310. Corresponding contacts 315 on the interior of the outer wallchamber 310 may also be provided, as shown. The gap region 316 betweenthe inner wall chamber 312 and the outer wall chamber 310 may contain avacuum or a gas, such as a tag gas that can be detected if the outerwall chamber 310 mechanically fails and potentially compromises themolten fuel storage. The gap region 316 between the outer wall chamber310 and the inner wall chamber 312 thermally isolates the inner wallchamber 312, such as from thermal communication with the coolant. Theflow temperature of the heat conducting fluid or coolant is typicallyless than the temperatures inside the inner wall chamber during anuclear reaction.

The multiple-walled fuel chamber 308 contains nuclear fuel 318 withinthe inner wall chamber 312. In the state shown in FIG. 3, the nuclearfuel 318 is in an initial state (e.g., at reactor start-up or initialinsertion into the nuclear reactor core). In one implementation, thenuclear fuel 318 is in a solid porous form including a combination offertile nuclear fuel and a bonding material (which may or may not beneutronically translucent). For example, the bonding material mayperform as a nuclear translucent carrier medium (e.g., when melted). InFIG. 3, the nuclear fuel 318 remains in a solid state because thetemperature of the nuclear fuel 318 has not exceeded its meltingtemperature (e.g., not satisfying a nuclear fuel melting temperaturecondition). Other example forms of the nuclear fuel materials mayinclude without limitation a solid non-porous slug, a powder, a slurry,and a suspension. For example, the fertile nuclear fuel may include ²³⁸U(uranium), such as a uranium foam, and the bonding material may includemagnesium (Mg). Other material types and structures may be employed. (Insome implementations, the nuclear fuel 318 may also include a quantityof fissile nuclear fuel, such as ²³⁹Pu (plutonium), particularly if thepassive reactivity control nuclear fuel device 300 is intended toundergo fission early in its life cycle (e.g., at startup).) In abreed-and-burn fast reactor, however, the fertile nuclear fuel caneventually be transmuted into fissile nuclear fuel that can undergofission.

A plenum region 320 is also located within the inner wall chamber 312 toreceive gaseous fission products as well as molten fuel as the fueltemperature rises and the fuel material expands into the plenum orplenum region 320. As shown, at least a portion of the plenum region 320is located outside the high neutron importance region. Thus, as moltenfuel expands into the plenum region 320 (and out of the high neutronimportance region), reactivity in the nuclear reactor core decreases.

One method of increasing the internal temperature of the nuclear fuel318 involves initiating and sustaining a nuclear fission reaction withinthe example passive reactivity control nuclear fuel device 300. Withinthe nuclear reactor core, neutrons within the nuclear reactor core cancollide with fissile nuclear fuel residing within the example passivereactivity control nuclear fuel device 300 to produce a nuclear fissionreaction sufficient to increase the internal temperature of the examplepassive reactivity control nuclear fuel device 300 to exceed the meltingtemperature of the bonding material. Once the bonding material is meltedinto a carrier material, the fissile nuclear fuel can go into solutionwith the carrier material to provide a fissile nuclear fuel solution(the molten fuel). Other methods of increasing the temperature of thenuclear fuel 318 may be employed.

FIG. 4A illustrates a perspective view of a passive reactivity controlnuclear fuel device 400 along a long axis 402, and FIG. 4B illustrates across-sectional view of the passive reactivity control nuclear fueldevice 400 along the long axis 402. In one implementation, an outerstructural wall 404 of the passive reactivity control nuclear fueldevice 400 forms a duct for containing a flow of coolant 406, such asliquid sodium. The outer structure wall 404 of the duct may bemanufactured from HT9 stainless steel, although other materials may beemployed.

Within the outer structural wall 404 is located a multiple-walled fuelchamber 408 having an outer wall chamber 410 and an inner wall chamber412. In one implementation, an outer wall chamber 410 of themultiple-walled fuel chamber 408 primarily includes HT9 stainless steel,although other materials may be employed, at least in part. Further, inone implementation, an inner wall chamber 412 of the multiple-walledfuel chamber 408 primarily includes molybdenum, although other materialsmay be employed, at least in part. In one implementation, the inner wallchamber 412 is fitted with one or more thermally conductive contacts,such as contact 414, which may also be formed from HT9 stainless steelor other materials. The contacts 414 can improve thermal communicationbetween the inner wall chamber 412 and the outer wall chamber 410 (andtherefore, the coolant 406) as one or more of the contacts 414 approachand/or physically contact the outer wall chamber 410. Correspondingcontacts, such as contact 415, on the interior of the outer wall chamber410 may also be provided, as shown. The gap region 416 between the innerwall chamber 412 and the outer wall chamber 410 may contain a vacuum ora gas, such as a tag gas that can be detected if the outer wall chamber410 mechanically fails and potentially compromises the molten fuelstorage. The gap region 416 between the outer wall chamber 410 and theinner wall chamber 412 thermally isolates the inner wall chamber 412,such as from thermal communication with the coolant. The flowtemperature of the heat conducting fluid or coolant is typically lessthan the temperatures inside the inner wall chamber during a nuclearreaction.

The inner wall chamber 412 contains nuclear fuel 418. In oneimplementation, the nuclear fuel 418 is in a solid porous form includinga combination of fertile nuclear fuel and a bonding material. Forexample, the fertile nuclear fuel may include ²³⁸U (uranium), such as auranium foam, and the bonding material may include magnesium (Mg). Othermaterial types and structures may be employed. (In one implementation ofthis initial state, the nuclear fuel 418 may also include a quantity offissile nuclear fuel, such as ²³⁹Pu (plutonium), particularly if thepassive reactivity control nuclear fuel device 400 is intended toundergo fission early in its life cycle (e.g., at startup).) In abreed-and-burn fast reactor, however, the fertile nuclear fuel caneventually be transmuted into fissile nuclear fuel that can undergofission.

FIG. 5 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device 500 containing molten fuel 501within a high neutron importance region of a nuclear reactor core. Asolid porous fuel slug as nuclear fuel 518 resides in the examplepassive reactivity control nuclear fuel device 500, such that the moltenfuel 501 can pass through the pores of the nuclear fuel 518. The passivereactivity control nuclear fuel device 500 includes a duct 504 throughwhich coolant 506, such as molten sodium, can flow. The duct 504 may bemanufactured from HT9 stainless steel although other materials may beemployed.

The duct 504 also contains a multiple-walled fuel chamber 508. In oneimplementation, an outer wall chamber 510 of the multiple-walled fuelchamber 508 primarily includes HT9 stainless steel, although othermaterials may be employed, at least in part. Further, in oneimplementation, an inner wall chamber 512 of the multiple-walled fuelchamber 508 primarily includes molybdenum, although other materials maybe employed, at least in part. In one implementation, the inner wallchamber 512 is fitted with one or more thermally conductive contacts514, which may also be formed from HT9 stainless steel or othermaterials. The contacts 514 can improve thermal communication betweenthe inner wall chamber 512 and the outer wall chamber 510 (andtherefore, the coolant 506) as one or more of the contacts 514 approachand/or physically contact the outer wall chamber 510. Correspondingcontacts 515 on the interior of the outer wall chamber 510 may also beprovided, as shown. The gap region 516 between the inner wall chamber512 and the outer wall chamber 510 may contain a vacuum or a gas, suchas a tag gas that can be detected if the outer wall chamber 510mechanically fails and potentially compromises the molten fuel storage.The gap region 516 between the outer wall chamber 510 and the inner wallchamber 512 thermally isolates the inner wall chamber 512, such as fromthermal communication with the coolant. The flow temperature of the heatconducting fluid or coolant is typically less than the temperaturesinside the inner wall chamber during a nuclear reaction.

The multiple-walled fuel chamber 508 contains nuclear fuel 518 withinthe inner wall chamber 512. In the state shown in FIG. 5, the nuclearfuel 518 is in an intermediate state in which the temperature of thenuclear fuel 518 has exceeded the melting temperature of the nuclearfuel 518 that has caused all or some portion of the nuclear fuel 518 toturn into molten fuel 501 (e.g., a solid portion of the fertile nuclearfuel can remain in a solid state). In one implementation, the nuclearfuel 518 can include a combination of a fissile nuclear fuel and acarrier material or a combination of fertile nuclear fuel, fissilenuclear fuel, and a carrier material. For example, the fertile nuclearfuel may include ²³⁸U (uranium), such as a uranium foam, and the bondingmaterial may include magnesium (Mg), and the fissile nuclear fuel mayinclude ²³⁹Pu (plutonium) in a Mg—Pu solution. Other material types andstructures may be employed. In a breed-and-burn fast reactor, however,the fertile nuclear fuel can eventually be transmuted into fissilenuclear fuel that can undergo fission.

It is noted that the nuclear fuel 518 includes solid fertile nuclearfuel and molten fissile fuel (e.g., a solution of fissile nuclear fueland the carrier material). The molten state of the fissile fueldemonstrates that the temperature of the nuclear fuel 518 has exceededthe melting temperature of the nuclear fuel 518 and the position of themolten fuel 501 within and not outside the high neutron importanceregion of the nuclear reactor core demonstrates that the temperature ofthe nuclear fuel 518 has not satisfied the negative reactivity feedbackexpansion temperature condition.

A plenum region 520 is also located within the inner wall chamber 512 toreceive gaseous fission products as well as molten fuel as the fueltemperature rises and the fuel material expands into the plenum orplenum region 520. As shown, at least a portion of the plenum region 520is located outside the high neutron importance region. Thus, as moltenfuel expands into the plenum region 520 (and out of the high neutronimportance region), reactivity in the nuclear reactor core decreases.

In one implementation, Mg may be used as the bonding material. Mg has amelting point of about 650° C. In contrast, one configuration of anuclear reactor includes an inlet coolant temperature of about 360° C.,which is generally insufficient to melt the Mg.

The internal temperature of the example passive reactivity controlnuclear fuel device 500, and therefore the bonding material, can beincreased to exceed the melting temperature of Mg, with some buffer toensure that the Mg is molten. The molten Mg forms a carrier material,which can act as a solvent for fissile nuclear fuel within the examplepassive reactivity control nuclear fuel device 500, such as ²³⁹Pu,providing a molten fissile nuclear fuel solution (Mg—Pu). One method ofincreasing the internal temperature of the nuclear fuel 518 involvessustaining a nuclear fission reaction within the example passivereactivity control nuclear fuel device 500. The increased internaltemperature of the nuclear fuel 518 can transition the nuclear fuel 518from a solid fuel state to a molten fuel state.

The fissile nuclear fuel may be initially stored within the inner wallchamber 512 of the example passive reactivity control nuclear fueldevice 500. Alternatively, or additionally, the fissile nuclear fuel maybreed up from the solid porous fertile nuclear fuel by fast neutronsresulting from fission reactions elsewhere within the operating nuclearreactor core.

FIG. 6 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device 600 containing molten fuel 601expanding outside a high neutron importance region of a nuclear reactorcore. A solid porous fuel slug as nuclear fuel 618 also resides in theexample passive reactivity control nuclear fuel device 600, such thatthe molten fuel 601 can pass through the pores of the nuclear fuel 618.The passive reactivity control nuclear fuel device 600 includes a duct604 through which coolant 606, such as molten sodium, can flow. The duct604 may be manufactured from HT9 stainless steel although othermaterials may be employed.

The duct 604 also contains a multiple-walled fuel chamber 608. In oneimplementation, an outer wall chamber 610 of the multiple-walled fuelchamber 608 primarily includes HT9 stainless steel, although othermaterials may be employed, at least in part. Further, in oneimplementation, an inner wall chamber 612 of the multiple-walled fuelchamber 608 primarily includes molybdenum, although other materials maybe employed, at least in part. In one implementation, the inner wallchamber 612 is fitted with one or more thermally conductive contacts614, which may also be formed from HT9 stainless steel or othermaterials. The contacts 614 can improve thermal communication betweenthe inner wall chamber 612 and the outer wall chamber 610 (andtherefore, the coolant 606) as one or more of the contacts 614 approachand/or physically contact the outer wall chamber 610. Correspondingcontacts 615 on the interior of the outer wall chamber 610 may also beprovided, as shown. The gap region 616 between the inner wall chamber612 and the outer wall chamber 610 may contain a vacuum or a gas, suchas a tag gas that can be detected if the outer wall chamber 610mechanically fails and potentially compromises the molten fuel storage.The gap region 616 between the outer wall chamber 610 and the inner wallchamber 612 thermally isolates the inner wall chamber 612, such as fromthermal communication with the coolant. The flow temperature of the heatconducting fluid or coolant is typically less than the temperaturesinside the inner wall chamber during a nuclear reaction.

The multiple-walled fuel chamber 608 contains nuclear fuel 618 withinthe inner wall chamber 612. In the state shown in FIG. 6, the nuclearfuel 618 is in an intermediate state in which the molten fuel 601 isheated to a high enough temperature that the molten fuel 601 expandswithin the inner wall chamber 612, into a plenum or plenum region 620,but not enough to cause a large enough increase in temperature to causethe inner wall chamber 612 to thermally expand significantly. In oneimplementation, the nuclear fuel 618 is in a solid porous form includinga combination of fertile nuclear fuel and a bonding material. Forexample, the fertile nuclear fuel may include ²³⁸U (uranium), such as auranium foam, and the bonding material may include magnesium (Mg). Othermaterial types and structures may be employed. (In one implementation ofthis intermediate state, the molten fuel 601 would also include aquantity of fissile nuclear fuel, such as ²³⁹Pu (plutonium) in a Mg—Pusolution.) In a breed-and-burn fast reactor, however, the fertilenuclear fuel can eventually be transmuted into fissile nuclear fuel thatcan undergo fission.

It is noted that the nuclear fuel 618 includes solid fertile nuclearfuel and molten fissile fuel (e.g., a solution of fissile nuclear fueland the carrier material). The molten state of the fissile fueldemonstrates that the temperature of the nuclear fuel 618 has exceededthe melting temperature of the nuclear fuel 618 and the position of themolten fuel 601 both within and outside the high neutron importanceregion of the nuclear reactor core demonstrates that the temperature ofthe nuclear fuel 601 has satisfied the negative reactivity feedbackexpansion temperature condition.

The plenum region 620 is also located within the inner wall chamber 612to receive gaseous fission products as well as molten fuel 601 as thefuel temperature rises and the fuel material expands into the plenum orplenum region 620. As shown, at least a portion of the plenum region 620is located outside the high neutron importance region. Thus, as moltenfuel expands into the plenum region 620 (and out of the high neutronimportance region), reactivity in the nuclear reactor core decreases.

FIG. 7 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device 700 containing molten fuel 701expanding outside a high neutron importance region of a nuclear reactorcore and expanding an inner wall chamber 712 within which it iscontained. A solid porous fuel slug as nuclear fuel 718 also resides inthe example passive reactivity control nuclear fuel device 700, suchthat the molten fuel 701 can pass through the pores of the nuclear fuel718. The passive reactivity control nuclear fuel device 700 includes aduct 704 through which coolant 706, such as molten sodium, can flow. Theduct 704 may be manufactured from HT9 stainless steel although othermaterials may be employed.

The duct 704 also contains a multiple-walled fuel chamber 708. In oneimplementation, an outer wall chamber 710 of the multiple-walled fuelchamber 708 primarily includes HT9 stainless steel, although othermaterials may be employed, at least in part. Further, in oneimplementation, the inner wall chamber 712 of the multiple-walled fuelchamber 708 primarily includes molybdenum, although other materials maybe employed, at least in part. In one implementation, the inner wallchamber 712 is fitted with one or more thermally conductive contacts714, which may also be formed from HT9 stainless steel or othermaterials. The contacts 714 can improve thermal communication betweenthe inner wall chamber 712 and the outer wall chamber 710 (andtherefore, the coolant 706) as one or more of the contacts 714 approachand/or physically contact the outer wall chamber 710. Correspondingcontacts 715 on the interior of the outer wall chamber 710 may also beprovided, as shown. The gap region 716 between the inner wall chamber712 and the outer wall chamber 710 may contain a vacuum or a gas, suchas a tag gas that can be detected if the outer wall chamber 710mechanically fails and potentially compromises the molten fuel storage.The gap region 716 between the outer wall chamber 710 and the inner wallchamber 712 thermally isolates the inner wall chamber 712, such as fromthermal communication with the coolant. Nevertheless, as the temperatureof the inner wall chamber 712 increases, the inner wall chamber 712 canthermally expand to overcome the thermal isolation. The flow temperatureof the heat conducting fluid or coolant is typically less than thetemperatures inside the inner wall chamber during a nuclear reaction.

The multiple-walled fuel chamber 708 contains nuclear fuel 718 withinthe inner wall chamber 712. In the state shown in FIG. 7, the nuclearfuel 718 is in a very high-temperature state, caused by a potentialcombination of neutron heating, gamma heating, and direction fission offuel material. At a sufficiently high temperature, the inner wallchamber 712 can thermally expand, such that the walls of the inner wallchamber 712 expand toward the wall of the outer wall chamber 710. In oneimplementation, the nuclear fuel 718 is in a solid porous form includinga combination of fertile nuclear fuel and a bonding material. Forexample, the fertile nuclear fuel may include ²³⁸U (uranium), such as auranium foam, and the bonding material may include magnesium (Mg). Othermaterial types and structures may be employed. (In one implementation ofthis intermediate state, the nuclear fuel 718 would also include aquantity of fissile nuclear fuel, such as ²³⁹Pu (plutonium) in a Mg—Pusolution.) In a breed-and-burn fast reactor, however, the fertilenuclear fuel can eventually be transmuted into fissile nuclear fuel thatcan undergo fission (e.g., a ²³⁸U fertile nuclear fuel material bred toa ²³⁹Pu fissile nuclear fuel material, which can go into solution withMg, designated as a Mg—Pu fissile fuel solution).

It is noted that the nuclear fuel 718 includes solid fertile nuclearfuel and molten fissile fuel (e.g., a solution of fissile nuclear fueland the carrier material). The molten state of the fissile fueldemonstrates that the temperature of the nuclear fuel 718 has exceededthe melting temperature of the nuclear fuel 718, and the position of themolten fuel 701 within and outside the high neutron importance region ofthe nuclear reactor core demonstrates that the temperature of the moltenfuel 701 has satisfied the negative reactivity feedback expansiontemperature condition. In addition, the expansion of the inner wallchamber 712 demonstrates that the temperature of the fuel and/or theinner wall chamber 712 satisfy an inner wall chamber expansion conditionin a very high-temperature condition. As the temperature of the nuclearfuel 718 decreases, the temperature may no longer satisfy the inner wallchamber expansion condition, such that the thermal expansion rate of theinner wall chamber 712 can also decrease and/or reverse.

A plenum region 720 is also located within the inner wall chamber 712 toreceive gaseous fission products as well as molten fuel as the fueltemperature rises and the fuel material expands into the plenum orplenum region 720. As shown, at least a portion of the plenum region 720is located outside the high neutron importance region. Thus, as moltenfuel expands into the plenum region 720 (and out of the high neutronimportance region), reactivity in the nuclear reactor core decreases.

In contrast to the state shown in FIG. 6, the state shown in FIG. 7depicts the molten fuel 701 at a sufficiently high temperature to forcethe inner wall chamber 712 to expand. In order to avoid mechanicalfailure of the inner wall chamber 712 when the temperature satisfies aninner wall chamber expansion condition (e.g., causing the inner wallchamber 712 to expand toward the outer wall chamber 710), the expansionmoves the contacts 714 of the inner wall chamber 712 toward the contacts715 of the outer wall chamber 710. As the distance between the contact714 and 715 decreases (particularly to the point of physical contact),the heat of the molten fuel 701 can radiate or conduct to the outer wallchamber 710, rapidly reducing the temperature of the molten fuel 701,even to the extent that the molten fuel 701 densifies rapidly, reducingthe temperature of the molten fuel 701, even potentially to the point offreezing the molten fuel 701 into a solid nuclear fuel slug of nuclearfuel 718. In this manner, an upper limit of the inner wall chambertemperature can be maintained—as the temperature increases, the innerwall chamber 712 thermally expands into radiative or conductive thermalcommunication with the outer wall chamber 710 and the coolant 706,resulting in a reduction in the temperature of the inner wall chamber712.

FIG. 8 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device 800 containing nuclear fuel 818within a high neutron importance region of a nuclear reactor core aftermolten fuel has densified. The passive reactivity control nuclear fueldevice 800 includes a duct 804 through which coolant 806, such as moltensodium, can flow. The duct 804 may be manufactured from HT9 stainlesssteel although other materials may be employed.

The duct 804 also contains a multiple-walled fuel chamber 808. In oneimplementation, an outer wall chamber 810 of the multiple-walled fuelchamber 808 primarily includes HT9 stainless steel, although othermaterials may be employed, at least in part. Further, in oneimplementation, an inner wall chamber 812 of the multiple-walled fuelchamber 808 primarily includes molybdenum, although other materials maybe employed, at least in part. In one implementation, the inner wallchamber 812 is fitted with one or more thermally conductive contacts814, which may also be formed from HT9 stainless steel or othermaterials. The contacts 814 can improve thermal communication betweenthe inner wall chamber 812 and the outer wall chamber 810 (andtherefore, the coolant 806) as one or more of the contacts 814 approachand/or physically contact the outer wall chamber 810. Correspondingcontacts 815 on the interior of the outer wall chamber 810 may also beprovided, as shown. The gap region 816 between the inner wall chamber812 and the outer wall chamber 810 may contain a vacuum or a gas, suchas a tag gas that can be detected if the outer wall chamber 810mechanically fails and potentially compromises the molten fuel storage.The gap region 816 between the outer wall chamber 810 and the inner wallchamber 812 thermally isolates the inner wall chamber 812, such as fromthermal communication with the coolant. The flow temperature of the heatconducting fluid or coolant is typically less than the temperaturesinside the inner wall chamber during a nuclear reaction.

The multiple-walled fuel chamber 808 contains nuclear fuel 818 withinthe inner wall chamber 812. In the state shown in FIG. 8, the nuclearfuel 818 is in a reset state (e.g., after sufficient heat has beenextracted from the nuclear fuel 818 to transition the molten fuel into asolid state, such as nuclear fuel 818). The temperature of the nuclearfuel 818 no longer exceeds the melting temperature of the nuclear fuel818 and no longer satisfies the negative reactivity feedback expansiontemperature condition. In one implementation, the nuclear fuel 818returns to a solid form including a combination of fertile nuclear fuel,fissile nuclear fuel, and a bonding material. For example, the fertilenuclear fuel may include ²³⁸U (uranium), such as a uranium foam, and thebonding material may include magnesium (Mg). Other material types andstructures may be employed. In a breed-and-burn fast reactor, however,the fertile nuclear fuel can eventually be transmuted into fissilenuclear fuel that can undergo fission.

A plenum region 820 is also located within the inner wall chamber 812 toreceive gaseous fission products as well as molten fuel as the fueltemperature rises and the fuel material expands into the plenum orplenum region 820. As shown, at least a portion of the plenum region 820is located outside the high neutron importance region. Thus, as moltenfuel expands into the plenum region 820 (and out of the high neutronimportance region), reactivity in the nuclear reactor core decreases.

FIG. 9 illustrates a cross-sectional view of an alternative examplepassive reactivity control nuclear fuel device. The passive reactivitycontrol nuclear fuel device may include a duct (not shown) through whichcoolant, such as molten sodium, can flow. The duct may be manufacturedfrom HT9 stainless steel although other materials may be employed.

In one implementation, a multiple-walled fuel chamber 900 includes anouter wall chamber 910 of the multiple-walled fuel chamber 900 thatprimarily includes HT9 stainless steel, although other materials may beemployed, at least in part. Further, in one implementation, an innerwall chamber 912 of the multiple-walled fuel chamber 900 primarilyincludes molybdenum, although other materials may be employed, at leastin part. A gap region 916 between the inner wall chamber 912 and theouter wall chamber 910 may contain a vacuum or a gas, such as a tag gasthat can be detected if the outer wall chamber 910 mechanically failsand potentially compromises the molten fuel storage. The gap region 916between the outer wall chamber 910 and the inner wall chamber 912thermally isolates the inner wall chamber 912, such as from thermalcommunication with the coolant. The flow temperature of the heatconducting fluid or coolant is typically less than the temperaturesinside the inner wall chamber during a nuclear reaction.

The multiple-walled fuel chamber 900 contains fertile nuclear fuel 918within the inner wall chamber 912, such as ²³⁸U (uranium) in a porous,powdered, or suspension form. The fertile nuclear fuel 918 is separatedfrom the fissile nuclear fuel region containing a liquid metal fuel 901,such as Mg—Pu, by a permeable barrier 919, although liquid metal fuel901 may reside on either side of the permeable barrier 919. As thefertile nuclear fuel 918 is transmuted into fissile nuclear fuel by fastspectrum neutrons resulting from fission reactions with the nuclearreactor (and potentially from within the multiple-walled fuel chamber900 itself), the transmuted fissile nuclear fuel diffuses into solutionwith the liquid metal fuel 901. In the state shown in FIG. 9, the liquidmetal fuel 901 is in an intermediate state (e.g., after sufficient heathas been provided to the liquid metal fuel 901 to maintain the moltenstate of the liquid metal fuel.

A plenum region 920 is also located within the inner wall chamber 912 toreceive gaseous fission products as well as molten fuel as the fueltemperature rises and the fuel material expands into the plenum orplenum region 920. As shown, at least a portion of the plenum region 920is located outside the high neutron importance region of a nuclearreactor core. Thus, as molten fuel expands into the plenum region 920(and out of the high neutron importance region), reactivity in thenuclear reactor core decreases.

FIG. 10 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device in which liquid metal fuel 1001has expanded outside a high neutron importance region of a nuclearreactor core. The passive reactivity control nuclear fuel device mayinclude a duct (not shown) through which coolant, such as molten sodium,can flow. The duct may be manufactured from HT9 stainless steel althoughother materials may be employed.

In one implementation, a multiple-walled fuel chamber 1000 includes anouter wall chamber 1010 of the multiple-walled fuel chamber 1000 thatprimarily includes HT9 stainless steel, although other materials may beemployed, at least in part. Further, in one implementation, an innerwall chamber 1012 of the multiple-walled fuel chamber 1000 primarilyincludes molybdenum, although other materials may be employed, at leastin part. A gap region 1016 between the inner wall chamber 1012 and theouter wall chamber 1010 may contain a vacuum or a gas, such as a tag gasthat can be detected if the outer wall chamber 1010 mechanically failsand potentially compromises the molten fuel storage. The gap region 1016between the outer wall chamber 1010 and the inner wall chamber 1012thermally isolates the inner wall chamber 1012, such as from thermalcommunication with the coolant. The flow temperature of the heatconducting fluid or coolant is typically less than the temperaturesinside the inner wall chamber during a nuclear reaction.

The multiple-walled fuel chamber 1000 contains fertile nuclear fuel 1018within the inner wall chamber 1012, such as ²³⁸U (uranium) in a porous,powdered, or suspension form. The fertile nuclear fuel 1018 is separatedfrom the fissile nuclear fuel region containing a liquid metal fuel1001, such as Mg—Pu, by a permeable barrier 1019, although liquid metalfuel 1001 may reside on either side of the permeable barrier 1019. Asthe fertile nuclear fuel 1018 is transmuted into fissile nuclear fuel byfast spectrum neutrons resulting from fission reactions with the nuclearreactor (and potentially from within the multiple-walled fuel chamber1000 itself), the transmuted fissile nuclear fuel diffuses into solutionwith the liquid metal fuel 1001. In the state shown in FIG. 10, theliquid metal fuel 1001 is in an intermediate state (e.g., aftersufficient heat has been provided to the liquid metal fuel 1001 tomaintain the molten state of the liquid metal fuel 1001) to furtherexpand the liquid metal fuel 1001 such that a considerable volume of theliquid metal fuel 1001 is located outside the high neutron importanceregion of the nuclear reactor core. By removing a considerable volume ofliquid metal fuel 1001 outside the high neutron importance region, thepassive reactivity control nuclear fuel device reduces reactivity withinthe nuclear reactor core.

A plenum region 1020 is also located within the inner wall chamber 1012to receive gaseous fission products as well as molten fuel as the fueltemperature rises and the fuel material expands into the plenum orplenum region 1020. As shown, at least a portion of the plenum region1020 is located outside the high neutron importance region of thenuclear reactor core. Thus, as molten fuel expands into the plenumregion 1020 (and out of the high neutron importance region), reactivityin the nuclear reactor core decreases.

It should be understood that an alternative implementation similar tothose illustrated in FIGS. 9 and 10 may not employ a permeable membrane.For example, a solid slug or a solid, porous slug of fertile materialmay be employed such that the fertile material generally maintains itsshape and location without the aid of a permeable membrane.

FIG. 11 illustrates a cross-sectional view of another alternativeexample passive reactivity control nuclear fuel device. The passivereactivity control nuclear fuel device may include a duct (not shown)through which coolant, such as molten sodium, can flow. The duct may bemanufactured from HT9 stainless steel although other materials may beemployed.

In one implementation, a multiple-walled fuel chamber 1100 includes anouter wall chamber 1110 of the multiple-walled fuel chamber 1100 thatprimarily includes HT9 stainless steel, although other materials may beemployed, at least in part. Further, in one implementation, an innerwall chamber 1112 of the multiple-walled fuel chamber 1100 primarilyincludes molybdenum, although other materials may be employed, at leastin part. A gap region 1116 between the inner wall chamber 1112 and theouter wall chamber 1110 may contain a vacuum or a gas, such as a tag gasthat can be detected if the outer wall chamber 1110 mechanically failsand potentially compromises the molten fuel storage. The gap region 1116between the outer wall chamber 1110 and the inner wall chamber 1112thermally isolates the inner wall chamber 1112, such as from thermalcommunication with the coolant. The flow temperature of the heatconducting fluid or coolant is typically less than the temperaturesinside the inner wall chamber during a nuclear reaction.

The multiple-walled fuel chamber 1100 contains liquid metal fuel 1101within the inner wall chamber 1112, such as Mg—Pu. In the state shown inFIG. 11, the liquid metal fuel 1101 is in an intermediate state (e.g.,after sufficient heat has been provided to the liquid metal fuel 1101 tomaintain the molten state of the liquid metal fuel), but the liquidmetal fuel 1101 has not expanded significantly outside the high neutronimportance region of the nuclear reactor core.

A plenum region 1120 is also located within the inner wall chamber 1112to receive gaseous fission products as well as molten fuel as the fueltemperature rises and the fuel material expands into the plenum orplenum region 1120. As shown, at least a portion of the plenum region1120 is located outside the high neutron importance region of thenuclear reactor core. Thus, as molten fuel expands into the plenumregion 1120 (and out of the high neutron importance region), reactivityin the nuclear reactor core decreases.

FIG. 12 illustrates a cross-sectional view of another alternativeexample passive reactivity control nuclear fuel device in which liquidmetal fuel 1201 has expanded outside a high neutron importance region ofa nuclear reactor core. The passive reactivity control nuclear fueldevice may include a duct (not shown) through which coolant, such asmolten sodium, can flow. The duct may be manufactured from HT9 stainlesssteel although other materials may be employed.

In one implementation, a multiple-walled fuel chamber 1200 includes anouter wall chamber 1210 of the multiple-walled fuel chamber 1200 thatprimarily includes HT9 stainless steel, although other materials may beemployed, at least in part. Further, in one implementation, an innerwall chamber 1212 of the multiple-walled fuel chamber 1200 primarilyincludes molybdenum, although other materials may be employed, at leastin part. A gap region 1216 between the inner wall chamber 1212 and theouter wall chamber 1210 may contain a vacuum or a gas, such as a tag gasthat can be detected if the outer wall chamber 1210 mechanically failsand potentially compromises the molten fuel storage. The gap region 1216between the outer wall chamber 1210 and the inner wall chamber 1212thermally isolates the inner wall chamber 1212, such as from thermalcommunication with the coolant. The flow temperature of the heatconducting fluid or coolant is typically less than the temperaturesinside the inner wall chamber during a nuclear reaction.

The multiple-walled fuel chamber 1200 contains liquid metal fuel 1201within the inner wall chamber 1212, such as Mg—Pu. In the state shown inFIG. 12, the liquid metal fuel 1201 is in an intermediate state (e.g.,after sufficient heat has been provided to the liquid metal fuel 1201 tomaintain the molten state of the liquid metal fuel), and the liquidmetal fuel 1201 has expanded significantly outside the high neutronimportance region of the nuclear reactor core. By removing aconsiderable volume of liquid metal fuel 1201 outside the high neutronimportance region, the passive reactivity control nuclear fuel devicereduces reactivity within the nuclear reactor core.

A plenum region 1220 is also located within the inner wall chamber 1212to receive gaseous fission products as well as molten fuel as the fueltemperature rises and the fuel material expands into the plenum orplenum region. As shown, at least a portion of the plenum region 1220 islocated outside the high neutron importance region of the nuclearreactor core. Thus, as molten fuel expands into the plenum region 1220(and out of the high neutron importance region), reactivity in thenuclear reactor core decreases.

Note that FIGS. 11 and 12 depict implementations that do not includefertile nuclear fuel within the inner wall chamber of themultiple-walled fuel chamber. While not explicitly described, otherimplementations of the described technology, such as that illustratedand described with regard to FIG. 3 may also be implemented withoutfertile nuclear fuel within the inner wall chamber of themultiple-walled fuel chamber.

FIG. 13 illustrates a cross-sectional view of an example passivereactivity control nuclear fuel device 1300 including an alternativelyshaped plenum region 1320 in an inner wall chamber 1312. Molten fuel1301 has expanded outside a high neutron importance region of a nuclearreactor core and into the plenum region 1320. A solid porous fuel slugas nuclear fuel 1318 resides in the example passive reactivity controlnuclear fuel device 1300, such that the molten fuel 1301 can passthrough the pores of the nuclear fuel 1318. The passive reactivitycontrol nuclear fuel device 1300 includes a duct 1304 through whichcoolant 1306, such as molten sodium, can flow. The duct 1304 may bemanufactured from HT9 stainless steel although other materials may beemployed.

The duct 1304 also contains a multiple-walled fuel chamber 1308. In oneimplementation, an outer wall chamber 1310 of the multiple-walled fuelchamber 1308 primarily includes HT9 stainless steel, although othermaterials may be employed, at least in part. Further, in oneimplementation, the inner wall chamber 1312 of the multiple-walled fuelchamber 1308 primarily includes molybdenum, although other materials maybe employed, at least in part. In one implementation, the inner wallchamber 1312 is fitted with one or more contacts 1314, which may also beformed from HT9 stainless steel or other materials. The contacts 1314can improve thermal communication between the inner wall chamber 1312and the outer wall chamber 1310 (and therefore, the coolant 1306) as oneor more of the contacts 1314 approach and/or physically contact theouter wall chamber 1310. Corresponding contacts 1315 on the interior ofthe outer wall chamber 1310 may also be provided, as shown. The gapregion 1316 between the inner wall chamber 1312 and the outer wallchamber 1310 may contain a vacuum or a gas, such as a tag gas that canbe detected if the outer wall chamber 1310 mechanically fails andpotentially compromises the molten fuel storage. The gap region 1316between the outer wall chamber 1310 and the inner wall chamber 1312thermally isolates the inner wall chamber 1312, such as from thermalcommunication with the coolant. The flow temperature of the heatconducting fluid or coolant is typically less than the temperaturesinside the inner wall chamber during a nuclear reaction.

The multiple-walled fuel chamber 1308 contains nuclear fuel 1318 withinthe inner wall chamber 1312. In the state shown in FIG. 13, the nuclearfuel 1318 is in an intermediate state in which the molten fuel 1301 isheated to a high enough temperature that the molten fuel 1301 expandswithin the inner wall chamber 1312, into a plenum region 1320, but notenough to cause a large enough increase in temperature to cause theinner wall chamber 1312 to thermally expand significantly.

It is noted that the nuclear fuel 1318 includes solid fertile nuclearfuel and molten fissile fuel (e.g., a solution of fissile nuclear fueland the carrier material). The molten state of the fissile fueldemonstrates that the temperature of the nuclear fuel 1318 has exceededthe melting temperature of the nuclear fuel 1318 and the position of themolten fuel 1301 both within and outside the high neutron importanceregion of the nuclear reactor core demonstrates that the temperature ofthe nuclear fuel 1301 has satisfied the negative reactivity feedbackexpansion temperature condition.

The plenum region 1320 is also located within the inner wall chamber1312 to receive gaseous fission products as well as molten fuel 1301 asthe fuel temperature rises and the fuel material expands into the plenumor plenum region 1320. As shown, at least a portion of the plenum region1320 is located outside the high neutron importance region. Thus, asmolten fuel expands into the plenum region 1320 (and out of the highneutron importance region), reactivity in the nuclear reactor coredecreases. The plenum region 1320 is shown as somewhat tapered, suchthat the diameter of the plenum region 1320 decreases as the distancefrom the high neutron importance region increases. Because the plenumregion 1320 has a smaller diameter than the inner wall chamber diameterwithin the high neutron importance region, the plenum region 1320 allowsthe molten fuel 1301 to expand farther away from the high neutronimportance region than a plenum region having a larger diameter, underthe same temperature conditions. Other plenum configurations arecontemplated.

An example passive reactivity control nuclear fuel device as described,for example, with regard to FIG. 2 represents device locatable within anuclear reactor core. In alternative implementations, however, a passivereactivity control nuclear fuel device can be manufactured on a largerscale to form a substantial volume of a nuclear reactor vessel. In suchan implementation, for example, a passive reactivity control nuclearfuel device can be manufactured such that the primary nuclear reactorcore is located within the inner wall chamber and molten fuel can bemoved out of a high neutron importance region within the inner wallchamber to a region of lower neutron importance region through thermalexpansion of the thermal fuel to provide negative reactivity feedback.Likewise, cooling the molten fuel such that the removed molten fuelmoves back into the high neutron importance region can provide positivereactivity feedback in such a large-scale passive reactivity controlnuclear fuel device.

The above specification, examples, and data provide a completedescription of the structure and use of exemplary implementations of theinvention. Since many implementations of the invention can be madewithout departing from the spirit and scope of the invention, theinvention resides in the claims hereinafter appended. Furthermore,structural features of the different implementations may be combined inyet another implementation without departing from the recited claims.

What is claimed is:
 1. A passive reactivity control nuclear fuel devicefor a nuclear reactor core, the passive reactivity control nuclear fueldevice comprising: a multiple-walled fuel chamber including an innerwall chamber configured to position nuclear fuel in a molten fuel statewithin a high neutron importance region of the nuclear reactor core, theinner wall chamber being further configured to allow at least a portionof the nuclear fuel to move in a molten fuel state into a lower neutronimportance region of the nuclear reactor core while remaining within theinner wall chamber as temperature of the nuclear fuel satisfies anegative reactivity feedback expansion temperature condition, themovement of the nuclear fuel in a molten fuel state to the lower neutronimportance region increasing negative reactivity feedback in the nuclearreactor core, and an outer wall chamber containing the inner wallchamber, a gap between the outer wall chamber and the inner wall chamberthermally isolating the inner wall chamber.
 2. The passive reactivitycontrol nuclear fuel device of claim 1 further wherein thermal isolationof the inner wall chamber contributes to maintaining the nuclear fuel ina molten state.
 3. The passive reactivity control nuclear fuel device ofclaim 1 further comprising: a duct containing the outer wall chamber andconfigured to flow a heat conducting fluid through the duct and inthermal communication with the outer wall chamber.
 4. The passivereactivity control nuclear fuel device of claim 1, wherein movement ofthe nuclear fuel in a molten fuel state to the lower neutron importanceregion of the nuclear reactor core decreases reactivity within thenuclear reactor core.
 5. The passive reactivity control nuclear fueldevice of claim 1, wherein the nuclear fuel is stored in a solid fuelstate within the inner wall chamber when temperature of the nuclear fueldoes not satisfy a nuclear fuel melting temperature condition.
 6. Thepassive reactivity control nuclear fuel device of claim 1, wherein themelting temperature of the nuclear fuel exceeds a flow temperature of aheat conducting fluid within a duct containing the outer wall chamber.7. The passive reactivity control nuclear fuel device of claim 1,wherein the inner wall chamber is expandable under increased internaltemperature of the inner wall chamber.
 8. The passive reactivity controlnuclear fuel device of claim 1, wherein the nuclear fuel includes asolid porous fertile nuclear fuel and a bonding material.
 9. The passivereactivity control nuclear fuel device of claim 1, wherein the nuclearfuel includes solid porous fertile nuclear fuel, a bonding material, andfissile nuclear fuel.
 10. The passive reactivity control nuclear fueldevice of claim 1, wherein the nuclear fuel in a molten fuel stateincludes a solution of fissile nuclear fuel and a nuclear translucentcarrier medium, the nuclear translucent carrier medium being formed froma melted bonding material.
 11. The passive reactivity control nuclearfuel device of claim 1, wherein the inner wall chamber is not inphysical contact with the outer wall chamber when the inner wall chamberhas not expanded under increased internal temperature of the inner wallchamber.
 12. The passive reactivity control nuclear fuel device of claim1, wherein the inner wall chamber radiates heat from within the innerwall chamber to a heat conducting fluid flowing outside the outer wallchamber when the inner wall chamber has expanded under increasedinternal temperature of the inner wall chamber.
 13. The passivereactivity control nuclear fuel device of claim 1, wherein the innerwall chamber conducts heat from within the inner wall chamber to a heatconducting fluid flowing outside the outer wall chamber when the innerwall chamber has expanded to physically contact the outer wall chamberunder increased internal temperature of the inner wall chamber.
 14. Thepassive reactivity control nuclear fuel device of claim 1, wherein theinner wall chamber conducts heat from within the inner wall chamber to aheat conducting fluid flowing outside the outer wall chamber when theinner wall chamber has expanded to allow physical contact between theouter wall chamber and contacts affixed to the inner wall chamber underincreased internal temperature of the inner wall chamber.
 15. Thepassive reactivity control nuclear fuel device of claim 1, whereintransfer of heat from the inner wall chamber to a heat conducting fluidflowing outside the outer wall chamber reduces the temperature of thenuclear fuel and transitions the nuclear fuel to a solid fuel state. 16.The passive reactivity control nuclear fuel device of claim 1, furthercomprising: one or more thermally conductive contacts affixed to theinner wall chamber and in thermal communication with the nuclear fuel,the thermally conductive contacts configured to physically contact theouter wall chamber when the inner wall chamber expands.
 17. The passivereactivity control nuclear fuel device of claim 1, wherein the innerwall chamber conducts heat from within the inner wall chamber to a heatconducting fluid flowing outside the outer wall chamber when the innerwall chamber has expanded to physically contact the outer wall chamberunder increased internal temperature of the inner wall chamber.
 18. Thepassive reactivity control nuclear fuel device of claim 1, wherein a gapbetween the inner wall chamber and the outer wall chamber is filled atleast in part by a tag gas.
 19. The passive reactivity control nuclearfuel device of claim 1, wherein the inner wall chamber includes a plenuminto which the nuclear fuel in a molten fuel state expands astemperature of the nuclear fuel in the molten fuel state increases. 20.A nuclear reactor having a nuclear reactor core comprising: a passivereactivity control nuclear fuel device located in the nuclear reactorcore, the passive reactivity control nuclear fuel device including amultiple-walled fuel chamber including an inner wall chamber configuredto position nuclear fuel in a molten fuel state within a high neutronimportance region of the nuclear reactor core, the inner wall chamberbeing further configured to allow at least a portion of the nuclear fuelto move in a molten fuel state into a lower neutron importance region ofthe nuclear reactor core while remaining within the inner wall chamberas temperature of the nuclear fuel satisfies a negative reactivityfeedback expansion temperature condition, the movement of the nuclearfuel in a molten fuel state to the lower neutron importance regionincreasing negative reactivity feedback in the nuclear reactor core, andan outer wall chamber containing the inner wall chamber, a gap betweenthe outer wall chamber and the inner wall chamber thermally isolatingthe inner wall chamber; and a duct containing the multiple-walled fuelchamber and configured to flow a heat conducting fluid through the ductand in thermal communication with the outer wall chamber.
 21. Thenuclear reactor of claim 20, wherein movement of the nuclear fuel in amolten fuel state into the lower neutron importance region of thenuclear reactor core decreases reactivity within the nuclear reactorcore.
 22. The nuclear reactor of claim 20, wherein the nuclear fuel isstored in a solid fuel state within the inner wall chamber whentemperature of the nuclear fuel does not satisfy a nuclear fuel meltingtemperature condition.
 23. The nuclear reactor of claim 20, wherein themelting temperature of the nuclear fuel exceeds a flow temperature ofthe heat conducting fluid within the duct.
 24. The nuclear reactor ofclaim 20, wherein the inner wall chamber is expandable under temperatureof the inner wall chamber.
 25. The nuclear reactor of claim 20, whereinthe inner wall chamber is not in physical contact with the outer wallchamber when the inner wall chamber has not expanded under increasedinternal temperature of the inner wall chamber.
 26. The nuclear reactorof claim 20, wherein the inner wall chamber radiates heat from withinthe inner wall chamber to the heat conducting fluid flowing outside theouter wall chamber when the inner wall chamber has expanded underincreased internal temperature of the inner wall chamber.
 27. Thenuclear reactor of claim 20, wherein the inner wall chamber conductsheat from within the inner wall chamber to the heat conducting fluidflowing outside the outer wall chamber when the inner wall chamber hasexpanded to physically contact the outer wall chamber under increasedinternal temperature of the inner wall chamber.
 28. The nuclear reactorof claim 20, wherein the inner wall chamber conducts heat from withinthe inner wall chamber to the heat conducting fluid flowing outside theouter wall chamber when the inner wall chamber has expanded to allowphysical contact between the outer wall chamber and contacts affixed tothe inner wall chamber under increased internal temperature of the innerwall chamber.
 29. The nuclear reactor of claim 20, wherein transfer ofheat from the inner wall chamber to the heat conducting fluid flowingoutside the outer wall chamber reduces the temperature of the nuclearfuel and transitions the nuclear fuel to a solid fuel state.
 30. Anuclear reactor having a nuclear reactor core comprising: amultiple-walled fuel chamber including an inner wall chamber configuredto contain the nuclear reactor core and to position nuclear fuel in amolten fuel state within a high neutron importance region of the nuclearreactor core, the inner wall chamber being further configured to allowat least a portion of the nuclear fuel to move in a molten fuel stateinto a lower neutron importance region of the nuclear reactor core whileremaining within the inner wall chamber as temperature of the nuclearfuel satisfies a negative reactivity feedback expansion temperaturecondition, the movement of the nuclear fuel in a molten fuel state tothe lower neutron importance region increasing negative reactivityfeedback in the nuclear reactor core, and an outer wall chambercontaining the inner wall chamber, a gap between the outer wall chamberand the inner wall chamber thermally isolating the inner wall chamber.